In many wireless networks, each node may have a need to determine the position of the node, relative to absolute and relative frames of reference. One system providing an absolute frame of reference is the Global Positioning System (GPS), using a constellation of geosynchronous satellites to provide the necessary coordinate reference points. Relative frames of reference are provided by asset tracking systems, such as those monitoring the movement of cooperating devices in a structure, for determining the building-relative position coordinates of a pre-mounted tagging device. Current systems for providing absolute and relative frames of reference typically require the cost and installation of infrastructure equipment, from satellites to fixed location beacons mounted in a structure, as used in many tracking systems.
Ad hoc wireless networks are characterized by the absence of installed infrastructure, and possibly unpredictable orientation and location of wireless devices in these networks. Such systems typically provide only a limited relative frame of reference, with respect to other devices in the ad hoc network. Certain ad hoc network nodes may be augmented with GPS to provide a fixed coordinate system, and hence an absolute frame of reference. However, the size and power requirements necessary for GPS are typically unavailable in energy-constrained ad hoc wireless devices. Also, the unpredictable orientation of the GPS antenna in a device, or a location without suitable GPS signals, may result in a failure to acquire sufficient GPS satellite signals for an accurate position determination by the node. Further, applications implemented with ad hoc wireless network may only require a relative, not an absolute, frame of reference, such as perimeter monitoring or surveillance.
In many position determination systems such as GPS, tagged asset tracking and ad hoc wireless networks, the propagation velocity of a radio frequency (RF) signal is ideally used to measure the line-of-sight distance between pairs of cooperating nodes. In air, the approximate velocity of a radio wave is one foot per nanosecond (1 ft./nSec), and the distance between a transmitter and one or more cooperating receivers may be measured using methods such as Time of Flight (TOF) or Time Difference of Arrival (TDOA). The results of such TOF ranging measurements between three nodes are required, at a minimum, for trilateration to determine the position of just a single other device in systems with some fixed infrastructure. More generally, many such TDOA ranging measurements are required to perform multilateration, such as required to determine the position of each device in an ad hoc wireless network. Therefore, the accuracy of such ranging measurements is a function of the clock precision utilized for each ranging measurement.
The use of conventional, or data modulated RF bands or channels, for measuring TOF or TDOA may produce inaccurate results due to various influences and effects. Multi-path RF propagation may result in the receiver detecting a higher strength, but not a line-of-sight path RF signal, and the measured time interval will have a larger value than for the line-of-sight RF wave path distance. Also, clock synchronization among the devices must be accomplished and maintained, typically through the use of a beacon signal transmitted by a designated node. Since such clocks are typically used for the purpose of providing a time base for demodulating data having a modulation frequency orders of magnitude lower than the carrier frequency, the clock is not able to resolve individual cycles of the carrier frequency. Hence, the precision of such ranging measurements is limited to an indirect detection of modulated data contained in a beacon signal, rather than arrival of a cycle at the carrier frequency. Further, each beacon signal will include a preamble appropriate for the data modulation technique employed. Since the signal quality at each receiver of the beacon signal may differ, the time required to achieve good synchronization with this preamble may differ from one receiver to another. As a result, the distance measurement will have an additional timing error due to the variation in response at each receiver, especially when multiple timing measurements are required, such as for trilateration and multilateration.
Some wireless data communications systems, such as conventional radio frequency systems, provide data communications by modulating, or coding, data signals onto a carrier frequency(s). However, other types of wireless communication systems are carrier-less and rely on time-based coding for data communications. One such communication system that relies on time-based coding to achieve reliable data communications is Ultra Wide Band (“UWB”). These UWB systems, unlike conventional radio frequency communications technology, do not use modulated carrier frequencies to transport data. Instead, UWB systems make use of a wide band energy pulse that transports data using both time-based coding and signal polarization. Time-based coding methods may include pulse-position, pulse-rate or pulse-width techniques.
UWB communication systems do not provide a common clock to the transmitting and receiving nodes. Instead, a low-drift clock with a programmable offset value is implemented in each transmitter/receiver node, providing a local reference for time-based coding and decoding. Each of these multiple clock domains is subject to short-term time drift, which will exceed the necessary tolerance for accurate UWB data communication system operation after a predictable time period. As a result, precise time synchronization between the transmitting node and receiving node(s) is imperative in UWB systems to obtain accurate data communications. In order to precisely synchronize one or more receivers (Rx) nodes with a transmitting node (Tx), UWB systems typically require preambles for each transmitted data frame. During the preamble, the programmable offset value is adjusted to minimize the error between the receiving node's clock and the clock used by the node transmitting the preamble. For some period of time after synchronization using a frame preamble the transmitter and receiver (or receivers) are able to exchange time-synchronous data.
At some point in time, the synchronization of the low-drift clocks will diverge sufficiently to increase the data error rate such that unreliable time-synchronous data communications occurs. A node must then transmit another frame preamble to enable the communicating nodes to re-synchronize their low-drift clocks for subsequent time-synchronous communications. Some applications with potential to benefit from UWB technology cannot be implemented if a preamble is required, due to the time and energy required for a preamble, during which no data is exchanged. Also, many potential applications for UWB technology are size and energy constrained, such as networks of wireless sensors and controls, which seek to minimize transmission time and to conserve energy.
While certain classes of applications require accurate timing to determine distances between nodes in a wireless sensor network (i.e. multi-static radar, geo-location, etc.) other applications exist for which a lower-precision timing capability is suitable for determining ranges (e.g. acoustic localization, seismic tracking, surveillance, etc.) For time-synchronized UWB data communications, each node's clock must have a precision in the sub-nanosecond range. This clock precision will be highest immediately following a time-synchronization preamble, then drift over time to a lower precision, until clock-synchronization is inadequate for UWB data communications. The lower precision timing range may be in the range of nano-seconds to milli-seconds, capable of supporting one or more of the class of applications requiring lower-precision timing. Since a wireless sensor network may be required to exchange data as well as measuring ranges, a method for achieving both, while minimizing energy consumption is essential to maximize the operational lifetime of a wireless sensor network node, many of which are battery-powered.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the communication industries for a method to provide collaboration among two or more transmitter/receiver nodes that utilizes the clock synchronization mechanism required for time-synchronous communication systems to provide ranging after the time-synchronous data exchange has been completed following a frame preamble.